The purpose of an ignition system is to ignite the compressed air/fuel mixture in proper time to thereby initiate the combustion process.
Though ignition systems used in the motor vehicle field have been strongly modified in a great many details over the past century, the basic principle has remained unchanged like that for the two most important engine concepts (gasoline and diesel engine). In case of the gasoline engine there is a high voltage of more than 25000 V generated which when applied to a spark plug causes a short-time arc discharge between electrode and ground.
While prior ignition systems comprised mechanical switches only it is customary practice today to use electronic transistor switches almost exclusively. Even though just one ignition coil and just one ignition distributor have been used conventionally, it becomes more and more customary to employ one ignition coil for each cylinder. Meanwhile one would be talking of a fully electronic ignition (VZ) because ignition triggering, ignition angle determination and distribution are all accomplished via electronic switches and/or components today. In modern microprocessor controlled electronic-map ignition systems it is that the ignition angle gets optimized dependent on speed and load. Information from many sensors such as the knock sensor, the motor temperature sensor and the throttle position sensor are provided for calculating the optimal ignition point in each case.
Most ignition systems are however based on the inductive principle for generating a high voltage by means of an ignition coil. The so-called high voltage capacitor discharge ignition system is an exception, but has substantially failed to make its way from a technical point of view. This concept also known as thyristor ignition is using a thyristor and a capacitor for pulse generation. An ignition transformer for high voltage generation is also used in practice. This concept, too, is employing a conventional spark plug. A drawback here resides in the fact that the spark duration is only 0.3 ms maximum so that in connection with a classic spark plug there is no guarantee for reliable ignition of the air/fuel mixture.
The basic principle of an ignition system using an ignition coil is a follows: Current from the battery and/or generator flows through the ignition coil primary winding to build up a strong magnetic field for energy storage when the breaker contact is closed with the ignition switch in the ON state. At the ignition point the breaker interrupts the current feed, the magnetic field energy as stored inside the coil attempts to keep up current supply and inside the secondary winding induces the high voltage needed for ignition which gets to the spark plug via coaxial high voltage cables to trigger an arc there. The energy required for this is in the range between 0.2 and 3 mJ. The ignition system carries stored energy to the order of 60 to 120 mJ in practice.
The electric signal getting to the spark plug is a so-called delta pulse under time range aspects. Since in practice the breaker contact cannot be opened infinitely fast either the mechanical or the electronic way and the ignition system (especially the extended ignition coil) is not capable of transmitting signals far into the GHz range, the signal involved here is a low pass limited signal. This means that the time signal has the characteristic of an SI pulse which is based on the function sin(x)/x. When considered under frequency range aspects, an ignition pulse has a very broad spectrum that theoretically starts at 0 Hz and in practice increases to higher frequencies in the three-digit MHz range and strongly decreases in the GHz range.
To sum up it is an objective by means of optimal ignition to achieve optimization in the sense of bringing up the engine to maximum power and/or securing minimum fuel consumption and/or obtaining exhaust gas purity while at the same time avoiding engine knocking.
Responsible for this in the end are the position, the form (length) and the duration of the arc on the spark plug. It is due to the electronic-map ignition now that the arc point precisely controllable while the other three parameters are substantially dependent on the configuration of the spark plugs and also on the architecture and the capability of the ignition system.
So-called ignition chambers or two spark plugs in the cylinder head are occasionally adopted to improve the configuration of the spark plug and/or combustion. The longer the distance between spark plug electrode and ground is the longer will be the ignition spark also. Provision of several ground arches on a spark plug permits to implement several spark gaps. One would maximize spark length and number of sparks to optimize combustion. This requires a higher voltage and power within the ignition system.
The ignition spark ignites the air/fuel mixture. The spark duration is substantially determined by the flame propagation (combustion rate) vM. The combustion rate vs is between 20 and 40 m/s. This means that the time for one combustion process ts referred to a cylinder radius r, of 5 cm is about 25 ms. Advantageous in the sense of low fuel consumption and hence high efficacy is a short spark duration and (relative to the piston movement) a correct timing of heat liberation which latter may be optimized by electronic-map ignition including knock sensors.
Apart from this ‘classic’ state of art there is also a first paper already that goes in the direction of the HF ignition hereinbefore presented, namely [3] A Novel Spark Plug for Improved Ignition in Engines with Gasoline Direct Injection (GDI)′ by Linkenheil et al, IEEE Transactions on Plasma Science, Vol 33, No. 5, October 2005). This paper describes in detail the reasons why a classic injection system fails to produce adequate results in the case of gasoline direct injection engines. To overcome the problems there is a design proposed which provides for an inner conductor of a coaxial resonator to protrude into the cylinder space.
It is clearly being described under [3] also that plasma generation in increasingly compressed air such as in gasoline engines is requiring an increase of electric field strength.
Critical Aspects of Prior Art
Most of today's ignition systems are operating with an inductive ignition system (ignition coil) and one spark plug (for each cylinder).
The ignition spark(s) on the only one spark plug per cylinder is (are) disposed in the center of the cylinder. Spark duration is dependent on the cylinder radius. Modern engines are of short stroke design and for this reason have a relatively large cylinder radius. Efforts to improve the design of conventional spark plugs have not yet succeeded in creating an arc range that would be capable of reducing the spark duration by factors. If spark duration could be shortened to one third of what is customary at present it would be practicable to achieve a marked improvement of efficacy to thereby get to lower fuel consumption and/or higher capacity yield.
The ignition system operates with extremely high voltages which factor in particular inhibits to achieve a high degree of integration of the system and also requires a very great deal of effort and expense to develop system component parts made from top-quality materials.
Voltage insulation is one of several reasons why an ignition system is not configured strictly to high-frequency aspects (i.e. in an impedance-controlled way). This missing high-frequency suitability in turn calls for use of a higher voltage.
The ignition spark and/or arc as generated extends completely from the electrode right up to ground. Ionization of the gas (air/fuel mixture) takes place within a narrow space. It is via this ionized path that a short-time current of very high density flows. This punctually high current density tends to cause heavy wear to the spark plugs. Though evermore improved and expensive materials have been used especially for the electrode the service life of a spark plug is limited to between 50000 and 80000 km. Consequently, spark plugs need to be replaced quite frequently which causes higher and higher expenses particularly where modern ultracompact engines are concerned.
The ignition system if of relatively low efficacy. An essentially improved efficacy not only would reduce current consumption, but also involves substantially less power loss in the form of heat dissipation which in turn permits to achieve a design which is less expensive and which offers a higher degree of integration.
For high frequency ionization to be achieved it is necessary to have a substantially high electric field strength which ought to be generated from as little power as possible. According to the solution proposed under [3] there is no impedance transformation taking place in the feed line to the resonator which (as will be described hereinafter) would be detrimental to generation of a high-strength field. The concept of having an additional resonator protrude into the cylinder is not in conformity with what is current practice either. Alternative resonator configurations will be presented in the following.
In addition has the effect of varying resonance frequency not been taken into consideration here (will as well be described hereinafter). An inadequate description of the resonator's loaded quality as well as the missing of a three-dimensional field simulation are further factors that are contributing to an inadequate high-frequency output yield. Seen as a whole, this approach has led to a solution that requires a peak power of about 600 W for plasma generation in the motor vehicle engine and can hence be carried into effect with a great deal of effort and expense only.
All prior known ignition systems are employing a metallic electrode which needs to have a good thermal bond to the cylinder head in order to prevent excessive heat-up and melting thereof. Such a good heat dissipation results in a marked reduction of an ignition system's efficacy.
Achievable Benefits
This present invention relates to creating an ignition system which is based on a relatively narrow-banded high-frequency signal (in the three-digit MHz range and throughout the entire GHz range) as well as a broad and almost optionally designed arc range (ignition range) which is not extending to ground and whose spark duration (duration of ignition) is selectively adjustable. The spark plug still comprises one single electrode of optional design. Cylinder head and piston are forming the ground.
This high-frequency ignition system permits to create a type of spark plug which for instance comprises one double electrode and consequently have two ignition spark paths. It is possible even to provide the electrode in form of a ring (torus) whose radius is ⅔ that of the cylinder. Gas ionization is only around said ring. Arcs are generated around the entire ring which do not extend to ground (cylinder head or piston) and whose lengths are in the centimeter range. It is by means of that ignition spark that at equal combustion rate or velocity the spark duration can be reduced to one third. The duration of spark ignition is now adjustable. This brings about a marked improvement of engine efficacy. Since the spark is in the centimeter range it is possible to have the spark duration substantially reduced even further due to the shorter paths.
The higher the frequency of the ignition signal will be selected, the lower can be the voltage applied to the spark plug. In the lower GHz range already for which a large number of low-priced electronic components are available it is practicable dependent on the arc length desired in each case to reduce the voltage to one-digit kV values at maximum. This reduction of maximum voltage enables the invention to be carried into effect with materials and components whose costs are substantially lower.
The fact that just one and/or two narrow-banded high-frequency signals may be used it is very easy to provide a design which is suitable for high-frequency operation. Lambda/2 lines with all of their benefits may now be used for instance which means that the lines need not to have a desired wave impedance any longer. This makes it easier to design a spark plug which is for instance suitable for high-frequency use.
The electrode now radiates energy over several paths or a large area. The electromagnetic energy generates a high-frequency current around the electrode within the ionized region which due to heat-up is caused in an arc mode to give off radiation energy in the optical range. Energy emission from the electrode is hence no longer in the form of a current, but of an electromagnetic field. The electrode is not loaded by the spark (field) any more so that no special metal is needed for the electrode. The spark plug may hence be used for as long as the entire life of the motor vehicle.
In an effort to minimize turbulences in particular it is possible to design the electrode to have a cylindrical shape and similar to a classic spark plug to just slightly protrude into the cylinder space. Other than in case of a conventional spark plug, however, any ground electrode that is chiefly responsible for turbulences is omitted.
Highly integrated and lowest cost high-frequency power amplifiers for use in GSM mobile communication systems and handsets have efficacies of more than 50%. Short lines may be implemented with substantially no losses in the GHz range. It is hence practicable for a high-frequency ignition system to ensure a very good efficacy and hence highly integrated design solutions.
In contrast to what is being described under [3] the cylinder space is either wholly or in part forming the high-frequency resonator. This avoids effort and expense and permits to keep the combustion chamber substantially unchanged. Spark plug design, too, is substantially easier and similar to that of a conventional spark plug, thus offering a lot of practical benefits. Impedance transformers moreover create a markedly higher field strength which compared to [3] helps to substantially bring down the necessary high-frequency outputs. In addition, varying resonance frequencies are followed up.
The materials selected for making electrodes include both metals and dielectric materials. An electrode may for example be composed of a ceramic material having a high dielectric constant and a very high melting point. Very efficient heat dissipation is hence no longer required such that a markedly improved efficacy may be achieved.